Cathode ray tube



Sept. 10, 1957 H. B. LAW 2,806,165

' CATHODE RAY TUBE Filed Aug. 18. 1954 INVENTOR. flA/Paw 5. LAW.

United States Patent 2,806,165 CATHODE RAY TUBE Harold ll Law,Princeton, N. 5., assignor to Radio Corporation of America, acorporation of Delaware Application August 18, 1954, Serial No. 450,665

7 Claims. (Cl. 31382) This invention relates to improvements incathode-ray tubes and particularly to means to prevent the loss ofcontrast in cathode-ray tubes, and means to prevent color dilution incathode-ray tubes utilized for color television reception.

Although the invention is applicable to cathode-ray tubes of the typeutilized in the reception of both monochrome and color televisionpictures, the invention will be explained, for purposes of brevity, withparticular examples of color television tubes only. One type of colortelevision tube comprises a color screen, or target, having an orderlyarray of narrow phosphor lines, or small phosphor dots, arranged ingroups of three colors, usually red, blue and green. The target assemblyincludes an apertured shadow mask through which electrons travel as abeam, or beams, in their transit to the tri-color screen. The particularline-like or dot-like color phosphor that is illuminated at any giveninstant is determined by the angle at which the scanning beam, or beams,pass through the apertured shadow mask and approach the screen. Shouldthe beam, or the electrons derived therefrom, strike a color phosphorother than one toward which the beam was originally directed, the purityof the color of the emitted light will beadversely affected. Thisphenomenon is known as color dilution. Color dilution may appear to theobserver either as a weakening of the selected color such that the hueremains the same or there may be an admixture in which another colorpartially replaces the desired color, i. e., red for blue.

Also, in the color kinescope tubes, as well as in the monochrome type ofkinescope tubes, the contrast of the picture obtained is deteriorated byreflection of ambient light from the kinescope face and by the effect ofscattered electrons striking the phosphor screen. In the latter case thescattered electrons may originate from the Walls of the envelope whenthe electron beam overscans the phosphor screen and strikes the innerwalls of the envelope. These scattered electrons from the envelope wallsmay strike the phosphor screen and cause a general illumination of thescreen which limits the black level that may be obtained.

It has been observed that the principal source of disturbing electronsis high velocity reflected primary electrons which are reflected fromthe envelope walls. These primary electrons are reflected whenoverscanning of the beam occurs particularly in the corners of thepicture where the phosphor screen is rounded oif but the raster is not.Generally speaking, any secondary'electrons, dislodged by the primarybeam overscanning, move in a random motion and at a low velocity whichis not sulficient to cause luminescence of'the phosphor particles.Therefore, this invention deals primarily with reflected, high velocity,primary electrons. It should be understood that the present invention isapplicable to color kinescopes of both the line and dot screen types andto monochrome kinescope tubes.

It is therefore an object of this invention to provide a Patented Sept.10, 1957 new and improved kinescope characterized, in operation, by'itssubstantial freedom from color dilution.

It is another object of this invention to provide a novel kinescopewherein high picture contrast is obtained.

These and other objects are accomplished in accordance with thisinvention by providing a cathode-ray tube comprising a sealed envelopehaving a novel shielding means therein. The envelope includes at leastone beam source of electrons in one end of the envelope and afluorescent screen in the other end of the envelope. The novel shieldingmeans includes an electron shield arranged adjacent to the source ofelectrons for collecting electrons which are overscanned. Adjacent tothe screen is provided a second electron shield for the purpose ofcollecting overscanned electrons that by-pass the first shield. By meansof the provision of two shields, the aperture in each of the shields islarge enough to permit useful output from the maximum area of the screenand small enough to collect overscanned and scattered electrons.

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself will best be understood by reference to the followingdescription when read in connection with the accompanying single sheetof drawings in which:

Fig. 1 is a transverse sectional view of a tri-color kinescope inaccordance with this invention;

Fig. '2 is an enlarged fragmentary elevational view of the screen unitof the color kinescope shown in Fig. 1;

Fig. 3 is a sectional view taken along line 33 of Fig. 1; and

Fig. 4 is an end view of an embodiment of this invention.

Referring now to Fig. 1 there is shown a sectional view of a colorkinescope tube 10. Although the invention is applicable to both colortubes, and monochrome type tubes, the invention will be explainedutilizing the color tube 10 as an example. The color kinescope tube 10comprises an evacuated envelope 11 including a main chamber in the formof a frustum 12, the large end of which terminates in a window, or faceplate, 13 which is transparent. The small end of the frustum 12terminates in a neck portion 14 which, in this case, contains a batteryof three electron guns 15, 16 and 17 arranged apart, and substantiallyparallel to the long axis of tube It). As shown the envelope 11 iscomposed of glass. However, it should be understood that the envelope 11could also be made of metal. Also, the envelope is shown as being afrustum but could be substantially rectangular in cross section.

In the enlarged end of frustum 12 there is provided a masked target 18which includes red, blue, and green phosphor dots (see Fig. 2) which arearranged in a conventional hexagonal mosaic pattern on the inner, ortarget, surface of the glass face plate 13 to form a screen 22. Theglass face plate 13 may be of any desired shape, i. e., circular orrectangular, and curvature, i. e., flat or spherical. In the instantcase the glass face plate 13 is in the form of a circular section of aspherical shell are sealed to the periphery of face plate 13. Any of thewell-known glass-to-metal seals may be utilized to form the seal betweenthe glass face plate 13 and mask 23, as

3 well as to seal the support frame 26 to the glass frustum 12.

The mask 23 may be made of a material having substantially a zerocoeflicient of thermal expansion over the normal operating range ofcolor kinescopes. One example of a metal having such a coetficient ofexpansion is Invar. Also, conductive materials which have highercoefficients of expansion may be utilized and temperature compensatingmeans employed. One example of the latter materials is a copper-nickelalloy of 95 percent copper and 5 percent nickel.

When, as in the instant case, the mask screen pattern is laid downdirectly on the curved face plate 13 of envelope 11, the mask 23 isappropriately curved so as to be approximately concentric with thecurved inner, or target surface, of face plate 13. As is well-known,apertures 24 in the mask 23 are arranged in the same hexagonal patternas the phosphor dots which are laid down on the face plate 13. In thepresent case the apertures 24 are of circular contour with substantiallya constant separation between their centers. With such a hexagonalarrangement of apertures 24 the red, blue and green scanning beams (seeFig. 1) pass through the plane of deflection P-P of the tube, producedby deflection yoke 19, with the beam centers equally spaced from eachother and at a common distance from the tube axis. The point in theplane of deflection from which each beam is apparently deflected iscalled the center-of-scan of that beam. The centers-of-scan for thethree beams are shown, in Figure 1, as dots in the plane of deflection.Since the electron beams from the three guns 15, 16 and 17 are focusedtoconverge into an aperture 24, it is obvious that the beams from thethree guns are not converged throughout the larger portion of thefrustum 12. Due to the fact that the beams are not converged in thefrustum 12, overscanning of the electrom beams results in differenteffects for each of the three beams. As an example, and assuming thatthe three beams are all overscanned to strike the walls of frustum 12,the angle of reflection for each of the beams will be slightlydifferent. Because of the difference in the angle of reflection betweenthe three beams the high velocity reflected primary electrons, arereflected at different angles.

In accordance with this invention the overscanning of the electron beamswhich, in the prior art, resulted in a loss of resolution, and in'colordilution, is overcome by the provision of an electron shield 28which is positioned within the enevelope 11 adjacent to the neck portion14 of envelope 11. Electron shield 28 comprises an apertured conductivemember which is supported within the envelope 11 by means of glassridges 29. The inner periphery of electron shield 28 is shaped in theconventional four to three aspect ratio and permits the beams from guns15, 16 and 17 to land on the entire useful area of target 18, whilepreventing reflected electrons from landing thereon.

The electron shield 28 may be any type of conductive material such ascarburized steel or aluminum and may be approximately of an inch thick.The shield 28 is preferably thin enough, and wit-h proper tensil properties, so that it may be slightly deformed to slide into ridges 29 on theenvelope 11. When the envelope 11 is made of glass, as in Fig. 1, it maybe desirable to make the shield 28 .of some type of non-magneticmaterial to avoid the possibility of shield 28 interfering with magneticlines of force from the deflection yoke 19. The electron shield 28preferably has an inner periphery, or aperture in the shape of a knifeedge toward the battery of electron guns so that a minimum of electronsfrom guns 15, 16 and 17 can be reflected from the thickness of theshield 28.

The shield 28 is provided for the. purpose of prevent ing electrons fromguns 15, 16 and 17 from landing on the inner walls of envelope 11 whenthebeams are overscanned. As is obvious, since the deflection centers ofthe electron beams from the guns 15, 16 and 17 are displaced one fromthe other, the beams from these guns are also displaced. Due to thedisplacement of the electron beams, and due to the fact that it isdesirable to scan the entire useful area of the screen 22, certainfactors must be considered in selecting an aperture size for the shield28. As an example, and since electron beams from guns 15 and 17 shouldscan the outer periphery of mask 23, there is a minimum diameterestablished for the size of the aperture in shield 28. However, sincethe guns 15, 16 and 17 are displaced the minimum size of the aperture inshield 28 is not sufiicient to collect electrons from all of the gunswhen the beams are overscanned (see beam 31). Therefore, a shield 30adjacent to the mask 23 is provided to collect any electrons whichbypass shield 28 and are reflected from the inner walls of the envelope11. Shield 30 is supported in the envelope 11 by glass ridges 32, andmay be similar in configuration and in composition to shield 28.

It may appear that the shield 28 can be omitted and all electronscollected from the walls of envelope 11 by means of shield 30. However,it has been found that there are many high velocity electrons which arereflected from the envelope walls, particularly in the corners of theraster, at such an angle that these reflected electrons would miss asingle shield such as shield 30. As is obvious if a single shield isutilized and the aperture therein is made small enough to collect allreflected electrons, some of the useful area of screen 22 will also beshielded. Therefore, in accordance with this invention, two shields,such as shields 28 and 30, are preferably utilized with shield 28adjacent to the deflection plane and shield 30 adjacent to the mask. Asshown in Figure l'both of the shields 28 and 30 extend inwardly to apoint that is substantially on a line between the centerof-scan and theoutermost of the apertures in mask 22.

As shown more clearly in Fig. 3 shield 30, as well as shield 28,includes an opening, or inner periphery, 27 which is shaped in theconventional four to three aspect ratio. Due to the location of shields28 and 30, the vast majority of electrons which are overscanned onto theenvelope walls are collected either by shield 28 or are deflected fromthe envelope walls and collected by shield 30. As is obvious to thoseskilled in the art, when a metallic envelope is utilized other means maybe provided for supporting the shields 28 and 30 such as L- shapedsupport members (not shown) which may be welded to the support membersand to the inner periphery of the envelope.

Referring now to Fig. 4 there is shown an embodiment of this inventionfor use in television tubes having a rectangular envelope. The shield 34substantially conforms to the form of the walls of a rectangularenvelope (not shown) and includes an aperture 35 which also is shaped inthe conventional four to three aspect ratio. The shield 34 may be madeof materials, and may be supported by means, similar to those describedin connection with Figures 1 through 3 so that further descriptionthereof is not deemed necessary.

What is claimed is:

1. A cathode-ray tube comprising an evacuated envelope, a beam source ofelectrons in one end of said envelope, means for producing acenter-of-scan for said electron beam, a target assembly comprising aviewing screen and an apertured electrically conductive mask throughwhich said beam passes in its transit to said screen, said screen beingsupported on the inner surface of one end of said envelope, a shieldingelectrode supported in said envelope adjacent to said center-of-scan forshielding said mask from electrons reflected from the inner walls ofsaid envelope and said shielding electrode extending inwardlysubstantially to a line between said center-of-scan and. the outermostof the apertures in said mask.

2. A cathode-ray tube as in claim 1 further comprising a secondshielding electrode supported adjacent to said mask and within saidenvelope.

3. A cathode-ray tube as in claim 1 wherein the outer periphery of saidshield Substantially conforms to the shape of said envelope and theinner periphery of said shield is substantially rectangular.

4. A cathode-ray tube comprising an evacuated envelope, at least onebeam source of electrons in one end of said envelope, means forproducing a center-ofscan for said electron beam, a target assembly inthe other end of said envelope and comprising a viewing screen and anapertured electrode, a first apertured electron shield adjacent to saidsource of electrons, a second apertured electron shield adjacent to saidtarget, said first and second electron shields extending inwardly fromthe walls of said envelope substantially to a line between saidcenter-of-scan and the outermost of the apertures in said aperturedelectrode, and said shields extending inwardly substantially normal tothe path of said beam.

5. A cathode-ray tube as in claim 4 wherein the outer periphery of eachof said shields substantially conforms to the shape of the innerperiphery of said envelope, and the inner periphery of each of saidshields is substantially rectangular.

6. A cathode-ray tube comprising an evacuated envelope including a neckportion and a conical portion, at least one source of electrons withinsaid neck portion,

means for producing a center-of-scan for said electrons, a targetassembly within the enlarged end of said conical portion, a firstapertured electron shield adjacent to the small end of said conicalportion, a second aperturedv References Cited in the file of this patentUNITED STATES PATENTS 2,123,957 Orth July 19, 1938 2,289,906 EpsteinJuly 14, 1942 2,580,697 Oliver Jan. 1, 1952 2,585,614 Bailey et a1 Feb.12, 1952 2,663,821 Law Dec. 22, 1953 2,682,620 Sanford June 29, 1954 IFOREIGN PATENTS 867,824 France Sept. 1, 1941

